为何ThreadLocal返回的值有相同的hashcode？

Question

最近在研究多线程，研究到ThreadLocal的时候，修改了一个参考的类，做了测试，发现竟然有的时候ThreadLocal所返回的值有相同的hashcode，百思不得其解，求解答。
下面是代码：
public class TreadLocalTest {
private static ThreadLocal seqNum = new ThreadLocal() {
public A initialValue(){
return new A();
}
};
public A getA(){
// seqNum.set(seqNum.get());
return seqNum.get();
}
public static void main(String[] args){
TreadLocalTest sn = new TreadLocalTest();
TestClient t1 = new TestClient(sn);
TestClient t2 = new TestClient(sn);
TestClient t3 = new TestClient(sn);
TestClient t4 = new TestClient(sn);
t1.start();
t2.start();
t3.start();
t4.start();
}
private static class TestClient extends Thread {
private TreadLocalTest sn;
public TestClient(TreadLocalTest sn){
this.sn = sn;
}
public void run(){
for(int i=0;i<3;i++){
System.out.println("thread["+Thread.currentThread().getName()
+"]sn["+sn.getA().hashCode()+"]");
}
}
}

}

这是另一个类，很简单，没有代码
public class A {
}
有时候测试，四个线程都不一样的hashcode，但是有时候，就会出现如下结果：
发现Thread-1，Thread-2所返回的A类竟然有相同的hashcode，为何呢？
thread[Thread-0]sn[28737396]
thread[Thread-0]sn[28737396]
thread[Thread-0]sn[28737396]
thread[Thread-1]sn[6927154]
thread[Thread-1]sn[6927154]
thread[Thread-1]sn[6927154]
thread[Thread-2]sn[6927154]
thread[Thread-3]sn[5442986]
thread[Thread-2]sn[6927154]
thread[Thread-3]sn[5442986]
thread[Thread-2]sn[6927154]
thread[Thread-3]sn[5442986]

Answer 1

我运行了你的代码，运行了很多次，出现过一次两个线程返回的A具有相同的hashCode，我也不清楚为什么。

代码结构可以调整一下，ThreadLocal直接放在Thread类中就可以了。

public class TreadLocalTest {

    private static class TestClient extends Thread {

        private ThreadLocal<A> seqNum = new ThreadLocal<A>() {
            public A initialValue() {
                return new A();
            }
        };

        public TestClient() {
        }

        public void run() {
            for (int i = 0; i < 3; i++) {
                System.out.println("thread[" + Thread.currentThread().getName()
                        + "]sn[" + seqNum.get().hashCode() + "]");
            }
        }
    }

    public static void main(String[] args) {
        TestClient t1 = new TestClient();
        TestClient t2 = new TestClient();
        TestClient t3 = new TestClient();
        TestClient t4 = new TestClient();
        t1.start();
        t2.start();
        t3.start();
        t4.start();
    }
}